It is known that the size and configuration of the holes in an underground soil drainage pipe can effect the performance of the pipe. For example, pipes with large holes are readily silted up if used in fine silt or sandy soils. Thus, a demand has arisen for different types of drainage pipes to be used under differing conditions. This has lead to a parallel demand for machinery capable of producing the various pipe hole configurations at high production speeds.
One known prior apparatus is described in U.S. Pat. No. 3,824,886 issued July 23, 1974 to Wilhelm Hegler. The apparatus disclosed in that patent includes a knife with a piercing point leading a concave cutting edge. In perforating a pipe, the piercing point moves into and through the pipe wall, the knife turns and the concave cutting edge moves tangentially through the pipe wall to cut a slot in it. The knife again turns to bring the piercing point through the pipe wall from the inside to the outside at the end of the slot. This piercing, slotting and severing action serves to chip a slot out of the pipe wall. To achieve the desired motion of the knife, it is mounted on a carrier that rotates about its own axis and revolves about the pipe. The rotation and revolution are so related that the leading end of the knife follows a path in the shape of an epitrochoid and chips slots from the pipe as it travels through nodes of the epitrochoid.
In another slot cutter described in the applicants'U.S. Pat. No. 4,180,357 issued Dec. 25, 1979, the knives are mounted on rotary but otherwise fixed spindles.
In all such slotters, the slot size is difficult to control as it varies widely with relatively small changes in other parameters such as the pipe wall thickness, knife adjustment and knife sharpness. This variance results primarily from the tapering of the ends of the slot as described, for example, in the Hegler Patent referred to above. Consequently, to provide for a more uniform opening size and a smaller opening size than is possible with a slot cutter, other types of machine have been developed which use a punching action in which a tool with a cutting end is driven radially through the pipe wall to form a perforation that is of substantially the same size and shape as the punching tool. Typical machines are described in Zieg et al U.S. Pat. No. 3,620,115 issued Nov. 16, 1971 and in Leloux German Pat. No. 2,652,169 of Oct. 2, 1980. While these punching type of perforators can produce holes of controlled size and shape, they require either an intermittent stopping of the pipe advance during the punching action or a complex mechanism to move the punching tool axially with the pipe as it is driven radially through the pipe wall.